This invention relates to molecular sieves and processes for their manufacture. More especially it relates to the preparation of synthesis mixtures to control product characteristics. The invention relates especially to the manufacture of silicoaluminophosphate molecular sieves, and more especially SAPO-34.
In U.S. Pat. No. 4,440,871, the preparation of a number of phosphorus-containing molecular sieves is described.
The patent particularly describes processes for the manufacture of numerous crystalline microporous silicoaluminophosphates (SAPO""s) including SAPO-34, employing sources of silicon (e.g., a silica sol), aluminium (e.g., hydrated aluminium oxide), and phosphorus (e.g., orthophosphoric acid), and an organic template, for example tetraethylammonium hydroxide (TEAOH), isopropylamine (iPrNH2) or di-n-propylamine (DPA). The patent, the disclosure of which is incorporated by reference herein, gives X-ray diffraction data for the SAPO""s and describes their utilities in catalysis and absorption.
International Application WO 00/06493 describes obtaining phosphorus-containing molecular sieves of lower particle size and narrower size distribution by agitation, e.g., stirring or tumbling.
EP-A-541 915 is concerned with the conversion of methanol to olefins (MTO), especially light (C2 to C4) olefins, using an aluminophosphate crystalline molecular sieve catalyst. The specification describes the advantages of small particle size catalysts in MTO processes, and provides a process for facilitating the manufacture of a small particle size material by stirring the synthesis mixture, producing SAPO-34 of median particle diameters, expressed as a mass distribution, in the range of about 0.6 to 1.4 xcexcm.
EP-A-185 525 describes a process in which SAPO-37 is manufactured using a two-phase synthesis mixture. In an example there are used an aqueous phase containing phosphoric acid, alumina, and tetraethyl and tetrapropyl ammonium hydroxides as organic templates, and an organic phase comprising tetraethyl orthosilicate in hexanol, a solvent immiscible with water.
International Application WO 01/36328 describes a process in which a SAPO-34 molecular sieve is manufactured in the form of isocrystalline spheroidal particles of from 0.5 to 30 xcexcm diameter using an aqueous synthesis mixture comprising a template, sources of the elements essential to the structure of the sieve and an organic solvent miscible with water, the purpose of the solvent being to solubilize the source of the silicon in the aqueous synthesis mixture, and a surfactant as morphology modifying agent. The silicon source may be a tetraalkyl orthosilicate.
The spheroidal particles of SAPO-34 are said to have a textured surface formed by crystallites the width (largest dimension) of which is from about 0.05 to 2.5 xcexcm, as determined by scanning election microscopy. Although it is stated that reducing the concentration of silicon in the synthesis mixture reduced the diameter of the spheroids, the product of the SAPO-34 examples is always spheroidal.
There remains a need, however, for a method whereby very small individual crystallites of SAPO-34 can be manufactured, as opposed to the clusters resulting from the process described in WO 01/36328.
It has now been found that by omitting the surfactant a SAPO-34 product in the form of individual crystallites of much reduced particle size may be obtained.
The present invention accordingly provides a process for the manufacture of a SAPO-34 crystalline molecular sieve which comprises forming a surfactant-free synthesis mixture containing sources of silicon, of aluminium, and of phosphorus, in proportions appropriate to the formation of SAPO-34, and a structure-directing agent, the source of silica being a tetraalkyl orthosilicate, and subjecting the synthesis mixture to hydrothermal treatment.
Advantageously, the molar ratio of silicon to aluminium, expressed as SiO2:Al2O3, is at most 0.5:1, since molar ratios higher than 0.5:1 are believed to contribute to the formation of a spheroidal particle as described in WO 01/36328.
The invention also provides SAPO-34, the mean particle size of which is at most 400 nm, advantageously at most 200 nm, preferably at most 100 nm and most preferably at most 50 nm. The SAPO-34 is advantageously one obtainable by, and preferably is obtained by, the process of the invention. Mean particle size may be measured by inspection of scanning electron micrographs (SEM""s), the largest dimension of each particle being taken. Alternatively, particle size measurement may be carried out by XRD peak width analysis as described in certain of the Examples below.
The components of the synthesis mixture used in the present invention are typically those known in the art or as described in the literature as suitable for the production of SAPO-34, as are the conditions of the hydrothermal treatment, except for the essential use of a tetraalkyl orthosilicate as the silicon source, the absence of surfactant and/or the required ratio of Al2O3:SiO2. (It may be noted that while U.S. Pat. No. 4,440,871 mentions the possibility of using a tetraalkyl silicate as silicon source, none of its many examples does so.)
Although the synthesis mixture is surfactant-free, it is within the scope of the invention to incorporate an organic solvent miscible with water, other than a surfactant, that assists in solubilizing the tetraalkyl orthosilicate in the aqueous synthesis mixture. As organic solvent there may be mentioned sulphoxides and C1 to C5 oxygenated hydrocarbons, the latter advantageously being an acid, aldehyde, ketone or mono- or polyhydric alcohol. As examples there may be mentioned ethylene glycol, 1,2- and 1,3-propanediols, acetone, methanol, n- and isopropanol, butanol and, preferably, ethanol.
In general, the treatment of the synthesis mixture to yield the desired crystalline molecular sieve, usually termed hydrothermal treatment, is advantageously carried out under autogenous pressure, for example in an autoclave, for example a stainless steel autoclave which may, if desired, be preferably lined. The treatment may, for example, be carried out at a temperature within the range of from 50, advantageously from 90, especially 120, to 250xc2x0 C., depending on the molecular sieve being made. The treatment may, for example, be carried out for a period within the range of from 1 to 200 hours, preferably up to 100 hours, again depending on the molecular sieve being formed. The procedure may include an ageing period, either at room temperature or, preferably, at a moderately elevated temperature, before the hydrothermal treatment at more elevated temperature. The latter may include a period of gradual or stepwise variation in temperature.
As source for the phosphorus in the synthesis mixture, there may be mentioned phosphoric acid, organic phosphates, e.g., triethylphosphate, and aluminophosphates.
As source for aluminium in the synthesis mixture there may be mentioned alumina hydrate, alumina, sodium aluminate, pseudoboehmite, aluminium phosphate, organic aluminium sources, e.g., alkoxides, for example, aluminium isopropoxide.
As source for silicon there may be mentioned, as indicated above, a tetraalkyl orthosilicate. There may be used, for example, tetramethyl orthosilicate, tetraethyl orthosilicate, tetrapropyl orthosilicate, and tetrabutyl orthosilicate. Tetraethyl orthosilicate (TEOS) is preferred.
In addition, the synthesis mixture will contain an organic structure-directing agent (template). In general, as indicated above, these compounds are generally organic bases, especially amines and quaternary ammonium compounds, used either singly or in mixtures.
As templates there may be mentioned, for example, tetraethyl ammonium hydroxide (TEAOH) and salts, e.g., phosphate, fluoride, chloride, bromide and acetate, dipropylamine (DPA), triethylamine, cyclohexylamine, 1-methylamidazole, morpholine, pyridine, piperidine, and diethylethanolamine (DEA).
The treatment may be carried out with the vessel static or, preferably, with stirring or with rotating the vessel about a horizontal axis (tumbling). If desired, the synthesis mixture may be stirred or tumbled during an initial part of the heating stage, for example, from room temperature to an elevated, e.g., the final treatment, temperature, and be static for the remainder. Agitation generally produces a product with a smaller particle size and a narrower particle size distribution than static hydrothermal treatment.
A synthesis mixture for producing SAPO-34 according to the invention advantageously has a molar composition, within the following ranges:
together with an organic template, advantageously tetraethylammonium hydroxide (TEAOH), dipropylamine (DPA), isopropylamine or morpholine, or a mixture of two or more such templates, in a proportion appropriate to yield SAPO-34.
The present invention also provides the use in the manufacture of SAPO-34 by hydrothermal treatment of a synthesis mixture comprising sources of silicon, aluminium and phosphorus, of a tetraalkyl orthosilicate as silicon source to control the particle size of the product.
The invention further provides the products of the processes and of the uses of the earlier aspects of the invention. The products, if required after cation exchange and/or calcining, have utility as catalyst precursors, catalysts, and separation and absorption media. They are especially useful in numerous hydrocarbon conversions, separations and absorptions. They may be used alone, or in admixture with other molecular sieves, in particulate form, supported or unsupported, or in the form of a supported layer, for example in the form of a membrane, for example as described in International Application WO 94/25151. Hydrocarbon conversions include, for example, cracking, reforming, hydrofining, aromatization, oligomerisation, isomerization, dewaxing, and hydrocracking (e.g., naphtha to light olefins, higher to lower molecular weight hydrocarbons, alkylation, transalkylation, disproportionation or isomerization of aromatics). Other conversions include the reaction of alcohols with olefins and the conversion of oxygenates to hydrocarbons, especially of methanol to olefins, especially light olefins. SAPO-34 produced by the process of the invention is especially suitable for this conversion.
Conversion of oxygenates may be carried out with the oxygenate, e.g., methanol, in the liquid or, preferably, the vapour phase, in batch or, preferably, continuous mode. When carried out in continuous mode, a weight hourly space velocity (WHSV), based on oxygenate, of advantageously 1 to 1000, preferably 1 to 100, hourxe2x88x921 may conveniently be used. An elevated temperature is generally required to obtain economic conversion rates, e.g., one between 300 and 600xc2x0 C., preferably from 400 to 500xc2x0 C., and more preferably about 450xc2x0 C. The catalyst may be in a fixed bed, or a dynamic, e.g., fluidized or moving, bed.
The oxygenate feedstock may be mixed with a diluent, inert under the reaction conditions, e.g., argon, nitrogen, carbon dioxide, hydrogen, or steam. The concentration of methanol in the feedstream may vary widely, e.g., from 5 to 90 mole per cent of the feedstock. The pressure may vary within a wide range, e.g., from atmospheric to 500 kPa.